Permeators containing hollow fiber membranes are characterized by having at least one end of each of the hollow fiber membranes embedded in a solid material in a fluid tight manner to form a tubesheet. This tubesheet is typically composed of crosslinkable epoxy resins which provide support for the hollow fibers in the permeator, mechanical integrity to withstand operating pressure and temperature conditions, chemical resistance, and imparts a tight seal to prevent fluid communication between the exterior side and the bore side of each of the hollow fiber membranes except through the walls of the hollow fiber membranes. There is a problem that tubesheets in hollow fiber permeators add significantly to the weight of the permeator assembly. Such weight is especially a concern in applications on board helicopters and aircraft, especially combat aircraft, where low weight is critical to high-performance operation. It is also necessary that the tubesheet be strong enough to withstand large shear stresses encountered when fluid pressure is applied to the tubesheet during operation of the permeator. The tubesheet and the hollow fiber membranes must also be easily machinable in order to facilitate the construction of the permeator.
It is known to put fillers in resins to reduce weight and increase strength, although these two properties do not commonly occur with the same filler. There is a problem when putting low weight fillers in the tubesheet because the fillers must mix thoroughly with the tubesheet resin, flow completely around the hollow fibers so as not to create voids in the tubesheet matrix which would decrease shear strength and create leak paths for the fluid through the tubesheet. The fillers must also maintain their integrity under the high temperatures often encountered in curing the commonly used polymerizable resins in tubesheets. U.S. Pat. No. 4,323,454 assigned to Monsanto Company describes a tubesheet which may incorporate a broad range of microsphere fillers for the primary purpose of reducing the peak curing temperature to fabricate the tubesheet. In particular, the tubesheet described in this patent has a resin containing a major amount by weight of polyglycidyl resin having a viscosity of about 1,000 to 30,000 centipoises at 25.degree. C.; curing agent composition in an amount sufficient to consume at least about 90 percent of the glycidyl groups in the liquid resinous composition; and particulate filler having a density of about 1 to 10 grams per cubic centimeter at 25.degree. C., an average maximum particle size of about 1.5 to 150 microns, and a surface area of less than about 5 square meters per gram of filler, said filler comprising about 5 to less than about 35 percent by volume of the liquid resinous composition. This reference discloses the use of fillers only for tubesheets made from a polyglycidyl resin, which resin has a viscosity of about 500 to 12,000 centipoises. It does not contemplate tubesheets having a resin with a higher viscosity, which resins are utilized in state-of-the-art tubesheets. Also, one of the shortcomings of the tubesheet in U.S. Pat. No. 4,323,454 is that it does not consider improvements to the machinability of the tubesheet while providing a lightweight, high strength tubesheet which maintains the desirable properties of the prior art. Machinability of the hollow fiber membranes is especially important in view of state-of-the-art hollow fiber membranes which are fragile and easily damaged during machining and skiving operations.
The prior art also discloses the use of microporous inorganic fillers in polymerizable compositions for dental applications which are smooth and easy to polish. The prior art also describes the production and use of thermoset plastic pellets containing expandable hollow microspheres. It is also known to use talc as an inorganic filler in tubesheets for hollow-fiber membrane permeators; however, the addition of talc can adversely effect the strength of the tubesheet.
The operating efficiency of a permeator depends partly on the ability of fluids to flow at the exterior and bore sides of the membranes. If fluid flow is restricted, a reduction in product recovery and/or purity may result. Therefore, it is highly desirable that the bores of the hollow fiber membrane be completely open for fluid communication through the tubesheet. The openings of the fiber bores can be easily obstructed during the preparation of the tubesheet. Specifically, these obstructions can occur from smearing and distortion of the fiber wall and epoxy matrix during the final skiving machining process of the tubesheet to open the fiber bores. Therefore, the filler should not adversely effect the machining of the tubesheet.
The present invention solves the problem of providing a light weight, high strength tubesheet which is easily machined and achieves maximum fluid flow through the opening of the fiber bores, without losing the desirable features of prior art tubesheets.